1. Field of the Invention
The present invention relates to a bumper reinforcing member which is a rigid member for an automobile bumper.
2. Description of the Related Art
Recently, most automobile bumpers are designed so that the bumper reinforcing member, which is a rigid member, is covered with a decorative plastic cover. This bumper has advantages. A performance of the rigid member is pursued without paying attention to the external appearance of the bumper reinforcing member itself, and the design of an automobile is enhanced by making the color of the plastic cover identical with that of the body of the automobile.
The bumper reinforcing member can be roughly classified into two types: a box-like section type and a closed cross-section type. The box-like section type is basically a tubular structure, and two sheet materials formed separately are connected to each other as disclosed in U.S. Pat. No. 5,722,708. In the closed cross-section type, one sheet material is roll-formed as disclosed in U.S. Pat. Nos. 5,306,058, 5,395,036 and 5,813,594.
In either bumper reinforcing member, the plastic cover is applied to a bumper front face, and a bumper rear face is connected to a bumper base of the automobile. A space formed inside the tubular structure absorbs and disperses a force at the time of a collision, thus protecting the automobile body, which is the main object of the bumper.
In order to further improve the structural strength of bumpers, there is an example in which a separate member such as beam reinforcing plate or bulkhead is attached to the bumper reinforcing member. This is taught by U.S. Pat. No. 5,425,561, for instance. This type of additional separate member is frequently seen in the box-like section type bumper reinforcing member. In addition, so as to improve the structural strength, there is another example in which an original sheet constituted by bonding a plurality of sheet materials different in kind or thickness is roll-formed so as to manufacture the bumper reinforcing member. This is shown in Japanese Patent Application Laid-Open No. 227331/1994.
The demands imposed on the bumper reinforcing member are (1) providing a sufficient structural strength (impact resistance) as the rigid member, (2) lightening an automobile in order to improve its fuel consumption and (3) keeping the manufacturing cost low. The demand (1) is often contradictory to the demands (2) and (3); and if the structural strength is increased, the lightening of an bumper and a reduction in the manufacturing cost become difficult. Therefore, the key is how the demands (2) and (3) are achieved while satisfying the demand (1).
First, the bumper reinforcing member cannot buckle at a time of a collision since this cannot protect the automobile body. Accordingly, for many bumper reinforcing members, it is necessary to add a separate member. As a result, the number of parts of the bumper reinforcing member and an additional step in the assembling process are inevitably unavoidable. In particular, if the assembling process is separately required, the advantage of roll forming that makes continuous manufacturing possible is impaired, thus increasing the manufacturing cost. Accordingly, it has been considered to increase the thickness of the sheet material instead of adding separate members. However, since this leads to a weight increase of the bumper reinforcing member, it is not preferable.